This disclosure relates in general to the monitoring of a fracturing operation, and to the characterisation of fracture networks in an Earth formation.
The characterisation of subsurface strata is important for identifying, accessing and managing reservoirs. The depths and orientations of such strata can be determined, for example, by seismic surveying. This is generally performed by imparting energy to the earth at one or more source locations, for example, by way of controlled explosion, mechanical input etc. Return energy is then measured at surface receiver locations at varying distances and azimuths from the source location. The travel time of energy from source to receiver, via reflections and refractions from interfaces of subsurface strata, indicates the depth and orientation of the strata.
Microseismic measurements can be characterized as a variant of seismics. In conventional seismic explorations a seismic source placed at a predetermined location, such as one or more airguns, vibrators or explosives, is activated and generates sufficient acoustic energy to cause acoustic waves to travel through the Earth. Reflected or refracted parts of this energy are then recorded by seismic receivers such as hydrophones and geophones.
In passive seismic or microseismic monitoring there is no actively controlled and triggered seismic source at a known location. The seismic energy is generated through so-called microseismic events caused by subterranean shifts and changes that at least partially give rise to acoustic waves which in turn can be recorded using suitable receivers. Although the microseismic events may be a consequence of human activity disturbing the subterranean rock, they are quite different from operation of equipment provided as an active seismic source. Background information on instruments and methods for microseismic monitoring can be found for example in U.S. Pat. Nos. 6,856,575, 6,947,843 and 6,981,550, published International Application Nos. WO 2004/0702424 and WO 2005/006020, and published United States Application No. 2005/01900649 A1, each of which are incorporated by reference herein for all purposes.
A specific field within the area of passive seismic monitoring is the monitoring of induced fracturing, where the fracturing may be induced by pumping a fluid, such as water or the like, at pressure into a borehole/wellbore (for purposes of this application the terms borehole, wellbore and well may be used interchangeably). Often induced fracturing is referred to as hydraulic fracturing as water is often the majority fluid used in the fracturing process. Such a hydraulic fracturing operation includes pumping large amounts of fluid to induce cracks in the earth, thereby creating pathways via which the oil and/or gas may flow. These cracks either will be new fractures created in previously continuous rock or will be along pre-existing faults and fractures. In general, the pathways induced by hydraulic fracturing operations will be a combination of newly created cracks and pre-existing faults and fractures. As and after a crack is generated, sand or some other proppant material is commonly injected into the crack to prevent it from closing completely when the fluid is no longer being pumped through the wellbore into the earth formation. The proppant particles that re placed within the newly formed fracture keep it open as a conductive pathway for the oil and/or gas to flow into the wellbore. In the hydrocarbon industry, hydraulic fracturing of a hydrocarbon reservoir may be referred to as “stimulation” as the intent is to stimulate the production of the hydrocarbons.
In the field of microseismic monitoring the acoustic signals generated in the course of a fracturing operation, which are caused by the generation of new cracks or displacement along existing cracks, are treated as microseismic events. Such microseismic events may occur as and after material is/has been pumped into the earth. Use may also be made of other information available from the fracturing operation, such as timing, flow rate and pressure. A well-known example of a set of microseismic data is the Carthage Cotton Valley data, evaluated for example by James T. Rutledge and W. Scott Phillips in: “HYDRAULIC STIMULATION OF NATURAL FRACTURES AS REVEALED BY INDUCED MICROEARTHQUAKES, CARTHAGE COTTON VALLEY GAS FIELD, EAST TEXAS”, Geophysics Vol. 68, No 2 (March-April 2003), pp. 441-452, and Rutledge, J. T., Phillips, W. S. and Mayerhofer, M. J., “FAULTING INDUCED BY FORCED FLUID INJECTION AND FLUID FLOW FORCED BY FAULTING: AN INTERPRETATION OF THE HYDRAULIC FRACTURE MICROSEISMICITY, CARTHAGE COTTON VALLEY GAS FIELD, TEXAS”, Bulletin of the Seismological Society of America, Vol. 94, No. 5, pp. 1817-1830, October 2004.
Microseismic monitoring of hydraulic fracturing is a relatively recent technology. In general, such monitoring is performed using a set of geophones located in a well in the proximity of the hydraulic fracturing. In microseismic monitoring, a hydraulic fracture is created down a borehole and data received from geophones, hydrophones and/or other sensors is processed to monitor the hydraulic fracturing. Typically the sensors are used to record microseismic wavefields generated by the hydraulic fracturing. By inverting the obtained microseismic wavefields, locations of microseismic events may be determined as well as uncertainties for the determined locations, source mechanisms and/or the like.
The spatial and temporal location of an induced microseismic event can be used to image the dynamics of a fracture growth and to quantify the stress regime in the reservoir together with formation and fault properties. This enables the effectiveness and efficiency of fracturing operations to be optimized by providing reliable information on the in-situ and induced reservoir parameters, together with the distribution of solid material within the induced pathways. Experimental work on core samples of rock (see for example Fredd, C. N., McConnell, S. B., Boney, C. L. and England, K. W. (2000): “EXPERIMENTAL STUDY OF FRACTURE CONDUCTIVITY FOR WATER-FRACTURING AND CONVENTIONAL FRACTURING APPLICATIONS”, Paper SPE 74138 presented at the 2000 SPE Rocky Mountain Regional/Low Permeability Reservoirs Symposium and Exhibition, Denver, Colo., 12-15 March) has shown that the conductivity of fractures is correlated to their width which in turn is strongly dependant on the type and amount of proppant within the fractures.
Recently the use of surface and/or shallow borehole seismic arrays has become more popular because of their economical efficiency. In surface and/or shallow borehole seismic surveys, unlike traditional downhole monitoring, it is possible to install tens, hundreds or even thousands of seismic sensors at the surface or at shallow depths. These generally provide superior azimuthal coverage of the energy radiated by microseismic events as compared to the coverage provided by one or two seismic arrays that are typically used in traditional downhole monitoring. However, at the same time, surface and/or shallow arrays tend to suffer from increased signal attenuation as a result of longer source and receiver distances, together with increased noise levels. Hence, improving the signal to noise ratio is a significant issue for improved event detection and characterization.